A base station of a radio communication system transmits signals having large differences between average power and peak power. In recent years, as means for achieving higher efficiency of transmission amplifiers that are used in the transmitters of such base stations, digital transmitters are being investigated that convert a transmission signal to a radio-frequency digital transmission signal and then amplify the signal, and switch-mode amplifiers such as class-D amplifiers or class-S amplifiers are being investigated as amplifiers. Assuming that the input signal is a pulse waveform signal, a switch-mode amplifier amplifies power while maintaining the pulse shape without alteration. The pulse waveform signal that has been amplified by a switch-mode amplifier is supplied from the digital transmitter after frequency components other than the band of the desired radio signal have been removed.
In addition, raising the purity and quality of the transmission signal necessitates conversion of a digital transmission signal to multi-bit signal.
As a means for raising the purity and quality of the transmission signal, a means is considered that, in order to support the conversion of a digital transmission signal to a multi-bit signal, provides a plurality of switch-mode amplifiers each corresponding to a respective bit of the plurality of bits of the digital transmission signal and that finally synthesizes the output signals of the plurality of switch-mode amplifiers to supply the synthesized signal to load.
However, none of Patent Documents 1 and 2 and Non-Patent Documents 1 and 2 that relate to transmitters that use switch-mode amplifiers discloses an actual signal-synthesizing circuit that synthesizes the output signals of a plurality of switch-mode amplifiers.